1. Field of the Invention
This invention relates to a system which provides a reliable communication link from an Electric Utility central control point, or the like, to a large number of customers for such purposes as electric power load management, meter rate setting, and remote meter reading. A synchronized technique using radio methods is disclosed having either one-way or optional two-way capability.
Utilities have expressed need for means to rapidly communicate signals en masse to or from their many customers for such purposes as selectively switching off non-vital appliances, (known as "load management"), for remotely setting meter rates, for remote setting of thermostats, for automatic meter reading, and for automating their power distribution system. The first three purposes require a one-way communication link from the power company to the user, while the second two purposes require bi-directional communiation means. The disclosed system provides both these "forward link" and "return link" roles.
The method herein disclosed employs one or more standard FM radio broadcast stations to transmit control signals to the customers and a low power narrow band transmitter at each customer location synchronized by the FM broadcast signal.
This invention relates to the subject matter of my U.S. Pat. No. 4,117,405, entitled, "Narrow Band Communication System," issued on Sept. 26, 1978.
The system disclosed here represents a significant departure from communication technologies which have heretofore been proposed for these applications. In the forward link the system employs one existing powerful FM broadcast station or several stations simultaneously to provide reliable coverage over more than 50,000 square miles at very low cost. In the reverse link the system uses a synchronous radio communication method which, for example, enables more than 500,000 separate power meter readings to be accomplished every hour, all on one single conventional radio channel.
2. Prior Art
The principal techniques competing to accomplish the communication functions required in load management and distribution automation include telephone, power line carrier communication, ripple control, radio, and various combinations of these.
Telephone methods are inherently attractive because a telephone line is frequently available to the controlled point. Unfortunately, the vast majority of U.S. telephone lines are incorporated in what is referred to as the "switched network" and because of this only a very small percentage of the telephones can be used at any one time. En masse communication is not possible without an enormous and expensive modification of the telephone plant. Futhermore, not all controlled points are accessable to existing phone lines and a significant number of new lines would be necessary for complete coverage. Of all competing systems, this is probably the most expensive.
An alternative system called "power line carrier", known for decades, uses the distribution power lines to carry signals and suffers many inherent problems arising from the numerous multiple paths, standing waves, and noise-like signals which can exist between utility and consumer. The greatest advantage of a power line carrier method is that the entire system may be owned and under the control of the power company. In general, the power line distribution system must be compensated and carefully checked a priori to insure reliable communication; this is both time consuming and expensive.
The third system, called ripple control, has been used successfully in many foreign countries as well as in a few applications in the United States. They also operate over power lines but only one-way and are relatively expensive installations because large and powerful signal injection equipment must be installed at each Utility substation. Furthermore, their data rate is necessarily low because these systems operate at very low carrier frequencies and require undesirably narrow signal bandwidth, consequently denying "instantaneous" communications. For example, well known existing systems take 20 seconds or more of communication time to affect a reliable one-way signal transmission.
Radio offers a fourth alternative but formidable problems exist for conventional radio systems which typically occupy a complete radio channel about 10 KHz wide during each transmission. Their companion forward link receivers must cope with relatively high radio noise levels and this, coupled with FCC transmitter power limitations, leaves much to be desired in the way of reliable transmission. Moreover, a private central radio transmitter erected by a power company to send control signals will generally be assigned a high radio frequency (e.g. VHF) and this results in relatively expensive control receivers for each consumer, when compared to the inexpensive receivers used in this invention.